Revealed: How pandemic swine flu kills

17:41 03 July 2009 by Debora MacKenzie

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As the H1N1 swine flu pandemic continues to spread around the world, most cases are still mild. But reports are starting to emerge of people who sicken and die very quickly of what appears to be viral pneumonia. Now two independent groups of scientists have now found out why – and it's all down to where the virus binds within the body.

H1N1 swine flu comes from pigs, so it binds well to cell-surface molecules in the respiratory tracts of other mammals, including humans. But there are slight differences in the way different flu proteins bind to these receptors.

Two separate teams – one led by Ron Fouchier at Erasmus University in Rotterdam, the Netherlands, and the other by Terrence Tumpey at the Centres for Disease Control in Atlanta, Georgia –

both report that the pandemic virus binds deeper than ordinary flu in the respiratory tract of ferrets, the animal most like humans when it comes to flu.

A virus of the same H1N1 family as the pandemic flu has been circulating as ordinary seasonal flu since 1977. Both groups found that the seasonal virus binds almost exclusively to cells in the ferrets' noses. But, the pandemic H1N1 binds deeper, in the lung's trachea, bronchi and bronchioles. The pandemic virus also replicated more, and caused more damage, though none of the ferrets were severely ill.

Pneumonia risk

Individuals differ in the way they react to viruses. A virus that binds deep in the lung can trigger potentially fatal pneumonia if the person infected mounts a strong inflammation in response to it..

The last H1N1 pandemic in 1918 was notorious for causing such rapid, viral pneumonia, which can kill within hours. "The binding and replication of the pandemic H1N1 virus in the lower respiratory tract in ferrets is consistent with the viral pneumonia that is observed in humans," Fouchier told New Scientist.

The US group found also found binding in the intestines, explaining the unusual nausea and vomiting seen in some cases of the pandemic flu.

Both teams concluded that the virus could adapt further to humans, which might make it more severe.

Journal reference: Science (DOI: 10.1126/science.1177238, DOI: 10.1126/science.1177127)